Fluid compositions which undergo a change in apparent viscosity in the presence of an electrical field are commonly referred to as electrorheological fluids or materials. Electrorheological materials normally are comprised of particles dispersed within a carrier fluid and in the presence of an electrical field, the particles become polarized and are thereby organized into chains of particles within the fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall fluid and in the absence of an electric field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the overall material is correspondingly reduced.
An electrorheological fluid composed of a non-conductive solid dispersed within an oleaginous fluid vehicle is described in U.S. Pat. No. 3,047,507. The compositions contain a minimum amount of water and a minimum amount of a surface active dispersing agent and the non-conductive solid consists of finely divided particles having an average diameter of from about 0.1 to about 5 microns.
A method of inducing a change in dynamic torque transmission of an electrorheological fluid in response to an electric field at low current is disclosed in U.S. Pat. No. 4,879,056. The method involves selecting a non-conductive liquid phase and dispersing in the liquid phase an alumino silicate particulate phase which is substantially free of adsorbed water. The resulting electrorheological fluid is then subjected to an electric potential in excess of about 1 kV at a current density of less than about 1/3 microamp per square inch.
U.S. Pat. No. 4,702,855 discloses electrorheological fluids consisting of an aluminum silicate solid dispersed within a fluid medium wherein the aluminum/silicate atomic ratio on the surface of the aluminum silicate is in the range of 0.15 to 0.80. The aluminum silicates may be either amorphous or crystalline and may contain contaminants such as Fe.sub.2 O.sub.3, TiO.sub.9, CaO, MgO, Na.sub.2 O, and K.sub.2 O. The electrorheological fluids may optionally contain an effective quantity of an appropriate dispersing agent.
As described above, the particles of electrorheological materials undergo polarization so as to be organized into chains of particles within the carrier fluid. The polarizability of particles traditionally utilized in electrorheological materials has typically been a function of the surface or bulk ionic conductivity of the particles themselves. Polarization resulting from surface or bulk ionic conductivity arises from the free migration of charged particles throughout the particle structure into positive and negative regions in response to an electric field. This type of particle conductivity has typically been achieved by the addition of an activator such as water to the particle component or by selecting a particle component which has a crystalline structure which enables cations to move freely through the structure. As a result, the electrorheological activity of many traditional electrorheological materials is dependent on the surface or bulk ionic conductivity of the particle component utilized in the overall material. This dependence on the ionic conductivity of the particle component causes the electrorheological material to be relatively sensitive to changes in temperature since both the mobility and concentration of charge carriers such as cations and anions are a function of temperature.
A need therefore exists for an electrorheological material which does not depend on the surface or bulk ionic conductivity of the particle component. Such an electrorheological material would be capable of functioning over a broad temperature range, which is desirable in many applications involving varying temperature conditions.